The invention relates to a hydraulic power lift mechanism, for an agricultural tractor or the like, of the kind having adjustable flow control means for enabling implements of different types and/or weights to be lowered at appropriate rates. Thus the rate of flow of oil to sump when lowering a heavy implement can be made considerably less than when lowering a light implement so that the rate of lowering is suited to the implement concerned. Although for a particular setting of the flow control means the rate of lowering is approximately the same for a light implement as for a heavy implement, the rate of lowering actually selected for a heavy implement, such as a fully-mounted multi-furrow one-way plough, must be considerably less than that selectively for a light implement because of the greater risk of damage to the heavy implement when it reaches the ground.
However, when the descent of a heavy implement is restrained by reducing the rate of flow of oil to sump, the implement reaches the ground without risk of damage, but is then slow to penetrate the ground to its required working depth. The object of the present invention is to avoid this disadvantage.